Human herpesvirus-8 (HHV-8) is associated with endothelial cell-derived Kaposi's sarcoma (KS) and B-cell proliferative diseases primary effusion lymphoma (PEL) and multicentric Castleman's disease (MCD). In each of these diseases, signalling by interleukin-6 (IL-6) and angiogenesis/vascular permeability promoted by vascular endothelial growth factor (VEGF) are believed to play key roles in disease development and progression. HHV-8 encodes a homologue of IL-6 (vlL-6) that can induce the expression of VEGF in endothelial and PEL cells and that is a mitogenic factor for these cells. Thus, vlL-6 is likely to be a contributory factor to HHV-8-associated disease, and represents a promising target for therapeutic drugs. The expression of vlL-6 at a very early stage of HHV-8 productive (lytic) replication and its ability to block IFN-mediated cell growth arrest and apoptosis suggests that the viral cytokine may also be a good target for antiviral therapy. However, there is presently no direct evidence about the role and importance of vlL-6 in lytic replication, nor has there been a systematic study to identify differences in signalling (STAT and MAPK) activated by endogenous IL-6 (hlL-6), that utilizes (IL-6R) and (gp130) receptor subunits, and vlL-6, that can signal through gpl30 alone. Our functional analyses of vlL-6 variants have provided evidence of fundamental differences in IL-6R-directed versus IL-6R-independent STAT signalling by vIL-6, and revealed corresponding differences in negative regulation of STAT signalling. We have also identified residues of vlL-6 important for receptor subunit interactions leading to signal transduction and generated specific antagonists of vlL-6/gpl30 signalling. The aims of the work proposed in this application are (1) to further define vlL-6:receptor interactions to allow us to engineer highly effective and specific vlL-6- and hlL-6 (non-immunogenic)-based antagonists of vlL-6 signal transduction, (2) to define qualitative differences in vlL-6 versus hlL-6 signal transduction and determine the influence of IL-6R on vlL-6 signalling and its regulation, and (3) to determine the role of vlL-6 in de novo infection and reactivated lytic replication of HHV-8 in cultured endothelial cells, the effects of vlL-6 on cellular gene expression in the context of viral infection, and the antiviral efficacies of our vlL-6/gpl30 signalling inhibitors. The proposed work will therefore examine the importance of vlL-6 and specific vlL-6-activated signalling pathways to virus biology and pathogenesis and generate potential antiviral and therapeutic agents designed to block vlL-6 function.